Over a decade ago, I worked in a big tech company that had a scheduled downtime on one Saturday a month. That was for database schema changes.
When you're changing the structure of how you keep track of customer data, you need to make sure that no customers are making changes at that same time. So you take the whole customer-facing service down for a little while, make the schema changes, test them, and then bring the customer-facing service back up. Ideally this takes a few minutes ... but you're prepared for it to take hours.
As the technology improved, and as the developers learned better how to make changes to the system without requiring deep interventions, long downtime for schema changes became less necessary ... for that particular business.
Every tech company pretty much has to learn how to do these sorts of changes for themselves, though.